Conventionally, as a system for desalinating seawater, there has been known a seawater desalination plant in which seawater passes through a reverse-osmosis membrane-separation apparatus to remove salinity from the seawater. In the seawater desalination system, the intake seawater is processed to have certain water qualities by a pretreatment system, and the pretreated seawater is delivered into the reverse-osmosis membrane-separation apparatus under pressure by a high-pressure pump. Part of the high-pressure seawater in the reverse-osmosis membrane-separation apparatus passes through a reverse-osmosis membrane against the osmotic pressure and is desalinated, and fresh water (permeate or desalted water) is taken out from the reverse-osmosis membrane-separation apparatus. The remaining seawater is discharged in a concentrated state of a high salt content as a concentrated seawater (brine) from the reverse-osmosis membrane-separation apparatus. The largest operational cost in the seawater desalination system is energy cost, and it depends heavily on energy for pressurizing the pretreated seawater up to such a pressure to overcome the osmotic pressure, i.e. up to the reverse-osmosis pressure. That is, the operational cost of the seawater desalination system is greatly affected by pressurizing energy of the seawater by the high-pressure pump.
Specifically, more than half of the electric power expenses as the highest cost in the seawater desalination system are consumed to operate the high-pressure pump for pressurizing the seawater. Then, pressure energy possessed by the high-pressure concentrated seawater with the high salt content which has been discharged from the reverse-osmosis membrane-separation apparatus is utilized for pressurizing part of the seawater. Therefore, as a means for utilizing the pressure energy of the concentrated seawater discharged from the reverse-osmosis membrane-separation apparatus to pressurize part of the seawater, there has been utilized an energy recovery chamber in which an interior of a cylinder is separated into two spaces by a piston arranged to be movable in the cylinder, a concentrated seawater port is provided in one of the two separated spaces to introduce and discharge the concentrated seawater, and a seawater port is provided in the other of the two separated spaces to introduce and discharge the seawater.
FIG. 13 is a schematic view showing a configuration example of a conventional seawater desalination system. As shown in FIG. 13, seawater pumped into the seawater desalination system by an intake pump (not shown) is processed to have certain water qualities by a pretreatment system 1 for removing suspended matter or the like, and then the pretreated seawater is branched via a feed pump 2 into a high-pressure pump line 3 and an energy-recovery-apparatus seawater supply line 4. The seawater which has flowed into a high-pressure pump 5 is pressurized by the high-pressure pump 5 and merges into the seawater pressurized by an energy recovery device 10 and a booster pump 7, and is then delivered under pressure to a reverse-osmosis membrane-separation apparatus 8.
Part of the seawater introduced into the reverse-osmosis membrane-separation apparatus 8 passes through a reverse-osmosis membrane (RO membrane) 8a of the reverse-osmosis membrane-separation apparatus 8 against the osmotic pressure and is desalinated, and desalted water is taken out through a desalted water line. The remaining seawater becomes a concentrated seawater with a high salt content, and the concentrated seawater is introduced into the energy recovery device 10 from the reverse-osmosis membrane-separation apparatus 8 through a concentrated seawater line 9.
In the energy recovery device 10, with the operation of a control valve 14, pistons 13, 13 are moved in the energy recovery chambers 11, 12 to introduce the seawater from the feed pump 2 through a check valve module 15 and to pressurize and discharge the seawater by utilizing the high-pressure concentrated seawater (reject).
The seawater pressurized in the energy recovery chambers 11, 12 is supplied to the booster pump 7 from the check valve module 15 through a booster-pump seawater supply line 6. With the booster pump 7, the seawater is pressurized by an amount corresponding to a pressure loss in the reverse-osmosis membrane-separation apparatus 8 and pipes, a pressure loss in the control valve 14, and a pressure loss generated in the energy recovery chambers 11, 12 and the check valve module 15, and the pressurized seawater merges into the seawater discharged from the high-pressure pump 5 and is then delivered under pressure to the reverse-osmosis membrane-separation apparatus 8.
In the above-described conventional energy recovery chamber, the piston in the energy recovery chamber is brought into sliding contact with the inner wall of the cylinder, and thus the sliding member of the piston is required to be periodically replaced due to wear of the sliding member. Further, the inner diameter of the long chamber is required to be machined with high accuracy so as to fit with the outer shape of the piston, and thus machining cost is very expensive.
Therefore, the applicants of the present invention have proposed an energy recovery chamber having no piston in which a cylindrical and elongated chamber is used as a pressure exchange chamber and a plurality of partitioned fluid passages are provided in the chamber to directly pressurize the seawater with the high-pressure concentrated seawater which is discharged from the reverse-osmosis membrane (RO membrane) in Japanese laid-open patent publication No. 2010-284642.